This application relates to a snubber circuit for dissipating voltage spikes downstream of a rectifier in a synchronous machine.
Synchronous generators are typically provided with either permanent magnet rotors, or synchronous rotors. In a synchronous rotor, a main field rotor winding is provided with electrical current from an exciter. The exciter consists of a exciter stator and exciter rotor. Both rotors are fixed to a shaft which is driven to rotate by a prime mover. The exciter rotor rotates adjacent an exciter stator, and the main field winding rotates adjacent a main stator. Injection of DC current into the exciter stator during rotation of the exciter rotor generates alternating voltage which is sent through a rectifier and then to the main field windings.
Rectifiers are often provided by a diode bridge. The rectifier's main function is to take the AC voltage from the exciter rotor, and transform it into DC power being sent downstream to the main field windings.
Due to the operation of the diode bridge, commutation of the diodes causes transient voltages on the rectifier DC terminals. The transient voltages can exceed the diode ratings and damage the diodes. Additionally, the transients associated with commutation can cause electromagnetic inference. Additionally, external events such as connecting generators in parallel and load transients can cause transient voltages across the diodes, which could damage the diodes.
To handle these various transients, suppression circuits have been utilized in such machines. In many machines, the suppression circuit has been provided by a resistor across the main field winding. The resistor generally reduces the amplitude of a voltage spike but does not substantially eliminate it. However, the resistors create a heat source, and there arises a tradeoff between the ohmic value of the resistor and the amount of heat generated by the resistor.
Other suppression circuits utilize a capacitor, which provides good dissipation of the spikes. However, when associated with the main field winding, which is effectively an inductor, steady voltage oscillations result, which may be undesirable.